The specific aim of this application is to study the molecular mechanisms which induce apparent negative cooperativity between macromolecules and between macromolecules and low molecular weight ligands. This study will be approached both experimentally and theoretically. The experimental approach is to study the kinetics of two negative cooperative systems: 1) the linkage between the self-association of glucagon and the binding of various ligands and 2) the binding of thyroxine and triiolothyronine by prealbumin. The primary techniques to be employed in this study are pressure jump, temperature jump and stopped flow. The theoretical aspects of this proposal is to postulate various new molecular mechanisms and then numerically test these mechanisms, and others from the literature, against our own experimental data and other data from the literature. The theoretical section is planned to include the experimental systems specified above and other systems of interest, such as the insulin receptor.